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Changing Time

StarDate:

October 18, 2011

If you fly from Los Angeles to Paris, the laws of physics will give you a bonus: you’ll be a few billionths of a second younger than if you’d stayed at home. But what physics gives, it also can take away. During the return trip, you’ll lose what you gained plus a wee bit more.

The changes are caused by an effect known as time dilation. It was predicted by Albert Einstein’s theories of Relativity. And it was verified 40 years ago this month, when two researchers took a set of atomic clocks on around-the-world plane trips — one going east, the other going west.

Relativity tells us that, as seen by an outside observer, a faster-moving clock will tick a little more slowly than a slower-moving clock, and that a clock in a strong gravitational field will tick more slowly than a clock in a weak one.

A clock in an airliner is farther from the center of Earth, so it “feels” a slightly weaker gravitational pull than one on the ground. And when you factor in Earth’s rotation on its axis, a clock that’s heading eastward is moving a little faster than one on the ground, while a clock that’s heading westward is moving a little slower than a clock on the ground.

In the 1971 experiment, the scientists predicted how much the time on the airborne clocks should differ from identical clocks on the ground. The experiment matched the predictions quite well. It confirmed what Einstein predicted: time is relative.

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